fn step_animations(&mut self) {
+    let mut rects_to_recompute_drawable_regions = vec![];
     for animation in self.ongoing_animations.iter() {
         let animation_damage = animation.step(now);
+        rects_to_recompute_drawable_regions
+            .push(animation_damage.area_to_recompute_drawable_regions);
     }
+    for r in rects_to_recompute_drawable_regions.drain(..) {
+        self.recompute_drawable_regions_in_rect(r);
+    }
 }